1. Field of the Invention
The present invention relates generally to voltage compensation techniques in an output driver for an integrated circuit. More particularly, the present invention relates to coupling a capacitive element between a drive transistor""s gate and a power supply and phasing-in portions of a voltage-supporting capacitance at slightly different times to smooth the compensating corrections.
2. State of the Art
As the sizes of semiconductor devices have reduced, so have the power supply voltages driving the devices. With smaller power supply voltages, respective signals within the semiconductor devices have also become smaller and more susceptible to variance caused by the influence of resistance, inductance, capacitance and switching within the semiconductor device. Unintended variances on a signal line can lead to an inability to correctly detect a signal and to detecting a signal when one was not intended. In either case, malfunctions and other signal errors may occur as a result of the variances.
As shown in FIG. 1, an output 2 of a semiconductor device conventionally includes an output driver 4 between the primary semiconductor circuitry 6 and the outputs 2. One purpose of the output driver 4 is to provide sufficient power compensation for the output signal to ensure the signal is output with an appropriate signal strength.
FIG. 2 shows a conventional output driver circuit 8. For a conventional semiconductor die, each output of the die is coupled to each of constant voltage signal lines envg less than 0 greater than  through envg less than 6 greater than  10, 12, 14, 16, 18, 20 and 22. Each of the constant voltage signal lines envg less than 0 greater than  through envg less than 6 greater than  10, 12, 14, 16, 18, 20 and 22 is further coupled to at least one output driver leg circuit 24, 26, 28, 30, 32, 34, 36, 38, 40, 42 and 44. Using one of the output driver leg circuits 44 coupled to envg less than 6 greater than  22 as an example, each output driver leg circuit conventionally includes a drive transistor 46 and a switching transistor 48.
The output driver circuit 8 shown in FIG. 2 is configured as an open-drain output driver. An open-drain configured output driver is one in which the drain of the drive transistor for each output driver leg circuit 24, 26, 28, 30, 32, 34, 36, 40, 42 and 44 is not coupled to circuitry within the semiconductor, but is directly coupled to an externally accessible contact, such as a bond pad. In a conventional open-drain configured output driver circuit 8 that has its output impedance controlled, such as that shown in FIG. 2, the drive transistor 46 will have its gate 50 set to a controlled voltage (typically 1.3 V to 1.4 V) such that the drive transistor 46 will be in saturation as much as possible while still achieving the required output drive. The switching transistor 48 is placed between the drive transistor 46 and a reference potential 53 such as a ground, to allow for switching the drive transistor 46 between off and on states.
The array of output driver leg circuits 24, 26, 28, 30, 32, 34, 36, 38, 40, 42 and 44 are conventionally configured such that the transistors used for the output driver leg circuits 32 and 40 coupled to constant voltage signal line envg less than 5 greater than  20 are approximately half the physical size of the transistors used for the output driver leg circuits 34 and 44 coupled to constant voltage signal line envg less than 6 greater than  22. Likewise, the transistors used for the output driver leg circuits 30 and 38 coupled to envg less than 4 greater than  18 are approximately half the physical size of the transistors used for the output driver leg circuits 32 and 40 coupled to constant voltage signal line envg less than 5 greater than  20. This pattern of using transistors approximately half the physical size of the transistors coupled to the next sequential envg less than  greater than  signal line continues down to the transistors coupled to envg less than 0 greater than  10. The physical size of the output driver leg circuit 42 is half the sum of the physical sizes of both output driver leg circuits 28 and 36. The output drive supplied by an output driver leg circuit is proportional to the physical size of the transistors used for that output driver leg circuit. By including output driver leg circuits, each providing a different output drive amount, a combination of different output driver leg circuits can provide a wide range of available output drive. Additional circuitry well-known to those of ordinary skill in the art determines how much output drive is needed for a particular output signal and controls which output driver leg circuits are switched xe2x80x9cONxe2x80x9d and xe2x80x9cOFFxe2x80x9d to provide an appropriate level of output drive.
Though there are many advantages to selectively switching the various output driver leg circuits 24, 26, 28, 30, 32, 34, 36, 38, 40, 42 and 44 xe2x80x9cONxe2x80x9d and xe2x80x9cOFF,xe2x80x9d the xe2x80x9cONxe2x80x9d and xe2x80x9cOFFxe2x80x9d action causes undesirable shifts in the drive transistor""s gate 50 voltage due to potential changes that couple back through the drive transistor""s gate 50. Namely, the drive transistor""s gate voltage may drop 100 mV from its desired level, for example, when the drive transistor 46 is switched to an xe2x80x9cONxe2x80x9d state which will reduce the output drive from its intended target. One method of compensating for this drop in voltage, as shown in FIG. 3, is to couple a capacitor 54 between the drive transistor""s gate 58 and a ground potential. With the capacitor in place, when the drive transistor 56 is turned xe2x80x9cONxe2x80x9d indirectly by the switching transistor 62, the voltage on the drive transistor""s gate 58 begins to drop toward a ground potential, but the capacitor 54, also referenced to the ground potential, reduces the voltage drop experienced. The larger the capacitor 54 used, the smaller the voltage dip caused when the drive transistor 56 is turned on. One example of a support circuit having capacitive support of this kind used in an output driver circuit may be found in Rambus Dynamic Random Access Memory (RDRAM) part 288MD-400-800, designed by Rambus, Inc. of Mountain View, Calif.
The repeated xe2x80x9cONxe2x80x9d-xe2x80x9cOFFxe2x80x9d action, with the voltage on the drive transistor""s gate 58 working to remain constant over a period of time, results in a square wave signal at the drive transistor""s gate 58. The output driver leg circuits"" control circuit then tries to set the DC average of the drive transistor""s gate 58 voltage equal to the desired voltage. As an example, using the output driver leg circuit of FIG. 3, if constant voltage signal lines envg less than 6 greater than  60 were set at an operating voltage of 1.4 V and the switching transistor 62 were turned on, the voltage on the drive transistor""s gate 58 would initially tend to be pulled down by the voltage on the drain of the switching transistor 62, perhaps to 1.3 V. The capacitor 54 on the drive transistor""s gate 58 of the drive transistor 56 would then tend to reduce the voltage drop experienced. If the switching transistor 62 were turned xe2x80x9cONxe2x80x9d and never turned xe2x80x9cOFF,xe2x80x9d the voltage on the gate 58 of the drive transistor 56 would eventually reattain 1.4 V due to the controlling circuit""s effect. For a typical output driver compensation circuit, however, the switching transistor 62 is not left xe2x80x9cON,xe2x80x9d but is repeatedly switched xe2x80x9cONxe2x80x9d and xe2x80x9cOFF.xe2x80x9d This toggling xe2x80x9cONxe2x80x9d and xe2x80x9cOFFxe2x80x9d results in a square wave signal on constant voltage signal lines envg less than 6 greater than  60 which eventually reaches a DC average of 1.4 V, toggling, for example, between 1.35 V and 1.45 V.
These variances in the voltage level of constant voltage signal lines envg less than 6 greater than  60 result in variances in the output signal power which can result in signal transmission errors. As a result, part specifications are used to identify the maximum allowable variance for reliable operation. For example, the specifications for the RDRAM 288MD-40-800, referenced previously, require that the voltage level on the drive transistor""s gate 58 have less than a 50 mV variance. To reduce the variance of the drive transistor""s gate 58 voltage to less than 50 mV requires a substantial capacitor 54 (approximately 15 times the physical size of the associated drive transistor 56) coupled to each envg less than  greater than  signal line. Where semiconductor space or xe2x80x9creal estatexe2x80x9d is precious, such a large capacitor for each output consumes a significant portion of the space available. It is therefore desirable to have an output drive circuit with minimal voltage variance without using such a large capacitor.
The present invention provides an output driver circuit for a semiconductor device, the output driver circuit including a plurality of compensating circuits which are capable of maintaining minimal variance in the drive transistor gate voltages without the use of excessively large capacitors. According to a first aspect of the present invention, a capacitor is coupled to the gate of a drive transistor and to a signal voltage. This signal voltage is established such that it opposes the voltage shift caused by switching the switching transistor xe2x80x9cON.xe2x80x9d By referencing the compensating capacitor to such a signal voltage, smaller capacitor sizes are needed to obtain the results offered by large capacitors referenced to ground. An additional capacitor may also be referenced to ground to provide additional compensation during the transitions of the switching transistor and the signal voltage.
According to a second aspect of the present invention, the voltage compensation provided to the gate of the drive transistor is added in phases, one portion at a time, to smooth out the response to the compensation correction. In one embodiment, rather than applying the compensation all at once, the compensation is added in two portions or phases. In another embodiment, the compensation is added in three phases. In yet another embodiment, the compensation is further supplemented with an additional capacitor referenced to a ground potential. In yet another embodiment, transistors having relatively long effective channel lengths are used to cause a delayed compensation to provide a fourth phase of compensation.
According to a third aspect of the present invention, existing output drivers are reworked to meet application specifications by reconnecting existing transistors within the semiconductor die to enable narrower specification parameters without large capacitors.
An electronic system is disclosed comprising a processor, a memory device, an input, an output and a storage device, at least one of which includes an output driver circuit with a compensating circuit having at least one capacitor coupled between the gate of a drive transistor and a signal voltage. A semiconductor wafer is also disclosed including an output driver circuit according to one or more embodiments of the present invention.